The present invention relates to a method of producing a molecular sieve containing sodium, lithium, rubidium, cesium, magnesium, calcium, strontium, silver and/or first row transition metal cations and partially or completely hydrolyzed trivalent metal cations, and more particularly to a method of producing a material containing a xe2x80x9ccontinuumxe2x80x9d of trivalent metal oxide by contacting zeolites with appropriate sources of trivalent cations and sodium, lithium, rubidium, cesium, magnesium, calcium, strontium, silver and/or first row transition metal cations. Specifically, the invention relates to the preparation of lithium ion-exchanged faujasite type zeolites containing a xe2x80x9ccontinuumxe2x80x9d of trivalent metal oxide.
Many industrially utilized zeolites are most economically synthesized in their sodium, potassium or mixed sodium-potassium cation forms. For example zeolites A, (U.S. Pat. No. 2,882,243), X (U.S. Pat. No. 2,882,244) and mordenite (L. B. Sand: xe2x80x9cMolecular Sievesxe2x80x9d, Society of Chemistry and Industry, London (1968), pages 71-76) are usually synthesized in their sodium forms, whereas zeolites LSX, i. e., zeolite X in which the framework silicon-to-aluminum atomic ratio is approximately 1, (UK 1,580,928) and zeolite L (U.S. Pat. No. 3,216,789) are usually synthesized in their mixed sodium and potassium forms. Zeolite L may also be readily synthesized in its pure potassium form.
Although these zeolites have useful properties in the as-synthesized form, it is often preferred to ion-exchange them to further enhance their adsorption and/or catalytic properties. This topic is discussed at length in chapter 8 of the comprehensive treatise of Breck (D. W. Breck: xe2x80x9cZeolite Molecular Sievesxe2x80x9d, Pub. Wiley, New York, 1973). Conventional ion-exchange of zeolites is carried out by contacting the zeolite, in either powdered or agglomerated form, using batch-wise or continuous processes, with aqueous solutions of salts of the cations to be introduced. These procedures are described in detail in Chapter 7 of Breck and have been reviewed more recently by Townsend (R. P. Townsend: xe2x80x9cIon Exchange in Zeolitesxe2x80x9d, in Studies in Surface Science and Catalysis, Elsevier (Amsterdam) (1991), Vol. 58, xe2x80x9cIntroduction to Zeolite Science and Practicexe2x80x9d, pages 359-390).
Conventional exchange procedures may be used economically to prepare many single and/or mixed cation exchanged zeolites. However, in the cases of lithium, rubidium and/or cesium exchange of sodium, potassium, or sodium-potassium zeolites, the original cations are strongly preferred by the zeolite; consequently, large excesses of expensive salts of the lithium, rubidium and/or cesium cations are needed to effect moderate or high levels of exchange of the original cations. Thus, these particular ion-exchanged forms are considerably more expensive to manufacture than typical adsorbent grades of zeolites. Furthermore, to minimize the cost of the final form of the zeolite, and to prevent discharge of these excess ions to the environment, considerable effort must be made to recover the excess ions from the residual exchange solutions and from washings in which the excess ions remain mixed with the original ions that were exchanged out of the zeolite. Since lithium-containing zeolites have great practical utility as high performance adsorbents for use in the noncryogenic production of oxygen, and rubidium and cesium exchanged zeolites have useful properties for the adsorptive separation of the isomers of aromatic compounds and as catalysts, this problem is of significant commercial interest.
U.S. Pat. No. 4,859,217 discloses that zeolite X (preferably having a framework silicon-to-aluminum atomic ratio of 1 to 1.25), in which more than 88% of the original sodium ions have been replaced by lithium ions, has very good properties for the adsorptive separation of nitrogen from oxygen. In the preparation of the zeolite, the base sodium or sodium-potassium form of the X zeolite was exchanged by conventional ion-exchange procedures, using 4 to 12 fold stoichiometric excess of lithium salts.
Additionally, a wide range of other lithium-containing zeolites allegedly exhibit advantageous nitrogen adsorption properties. For example, U.S. Pat. Nos. 5,179,979, 5,413,625 and 5,152,813 describe binary lithium- and alkaline earth-exchanged X zeolites; U.S. Pat. Nos. 5,258,058, 5,417,957 and 5,419,891 describe binary lithium- and other divalent ion-exchanged forms of X zeolite; U.S. Pat. No. 5,464,467 describes binary lithium- and trivalent ion-exchanged forms of zeolite X; EPA 0685429 and EPA 0685430 describe lithium-containing zeolite EMT; and U.S. Pat. No. 4,925,460 describes lithium-containing chabazite. In each case conventional ion-exchange procedures are contemplated, involving significant excesses of lithium cations over the stoichiometric quantity required to replace the original sodium and/or potassium cations in the zeolite. In the case of the binary lithium-exchanged zeolites, it may sometimes be possible to slightly reduce the quantity of lithium salt used by carrying out the exchange with the second cation before the lithium ion-exchange step (U.S. Pat. No. 5,464,467) or by carrying out both exchanges simultaneously (EPA 0729782), but in either case a large excess of lithium cations is still needed to achieve the desired degree of exchange of the remaining sodium and potassium cations.
U.S. Pat. No. 5,916,836, issued to Toufar et al., discloses a method of preparing lithium-exchanged or polyvalent cation- and lithium-exchanged molecular sieves from molecular sieves that originally contain sodium ions, potassium ions or both sodium and potassium ions without requiring the use of a large excess of lithium ions. The method of Toufar et al. includes the step of exchanging the original zeolite with a source of ammonium ions prior to the lithium ion-exchange. The initial molecular sieve may contain polyvalent cations in addition to sodium and/or potassium ions, or polyvalent cations may be introduced at any stage of the process.
This invention presents an efficient method of preparing zeolites with enhanced adsorption and catalytic properties, containing sodium, lithium, rubidium, cesium, magnesium, calcium, strontium, silver and/or first row transition metal cations, and partially or completely hydrolyzed trivalent metal cations. A principal advantage of the process of this invention is that it enables the preparation of these zeolites without using large excess of lithium, rubidium, cesium, magnesium, calcium, strontium, silver and/or first row transition metal ion sources.
According to a broad embodiment, the invention comprises a method of producing a cation-exchanged zeolite comprising the steps:
(a) contacting at least one synthetic zeolite selected from the group consisting of structure types FAU, EMT, LTA, CHA, MOR, OFF, ERI, zeolite ZK-5, BEA and GME and combinations thereof and containing sodium ions, potassium ions or mixtures thereof with a source of trivalent cations, thereby replacing most of the sodium ions, potassium ions or mixtures thereof with trivalent cations and producing a substantially trivalent cation-exchanged zeolite;
(b) calcining the substantially trivalent cation-exchanged zeolite at a temperature in the range of about 200 to about 650xc2x0 C., thereby generating protons in said zeolite; and
(c) contacting the calcined zeolite with a source of sodium, lithium, rubidium, cesium, magnesium, calcium, strontium, silver and/or first row transition metal hydroxides or mixtures thereof, or precursors thereof, thereby replacing protons and at least some of the initial cations remaining on the zeolite with sodium, lithium, rubidium, cesium, magnesium, calcium, strontium, silver and/or first row transition metal cations, or mixtures thereof.
In a preferred embodiment of the invention, the at least one synthetic zeolite contains sodium ions. In this preferred embodiment, the at least one synthetic zeolite preferably comprises type A zeolite, type X zeolite, type EMC-2 zeolite, mixtures of two or more of type A zeolite, type X zeolite, or type EMC-2 zeolite or intergrowths of two or more of type A zeolite, type X zeolite, or type EMC-2 zeolite. In a more preferred embodiment, the at least one synthetic zeolite comprises type X zeolite, and in a still more preferred embodiment, the at least one synthetic zeolite comprises type X zeolite having a framework silicon-to-aluminum atomic ratio of 0.9 to 1.1.
In another preferred embodiment of the invention, the trivalent cations comprise aluminum, gallium, iron, chromium, indium, single rare earth ions, mixtures of two or more rare earth ions, or mixtures thereof.
In a more preferred embodiment, the trivalent cations comprise at least one rare earth ion. In another preferred embodiment, rare earth cations comprise about 50 to about 150%, on an equivalents basis, of the total exchangeable cations on the zeolite. In this more preferred embodiment, step (b) of the broad embodiment is preferably carried out at a temperature in the range of about 250 to about 550xc2x0 C. In a still more preferred embodiment, the at least one rare earth ion comprises about 50 to about 100%, on an equivalents basis, of the total exchangeable cations on the zeolite.
In another preferred embodiment of the invention, the calcined zeolite is contacted with a source of lithium ions in step (c). Preferably, in this embodiment the lithium ions are in the form of lithium hydroxide or a precursor thereof, and more preferably, the reaction zone is an aqueous medium. In a preferred aspect of this preferred embodiment, step (c), is carried out at a temperature in the range of about 0 to about 100xc2x0 C. In another preferred aspect of this preferred embodiment, the lithium ion exchange step is carried out at a pH value greater than about 10.
In another preferred embodiment, the at least one synthetic zeolite initially contains sodium ions, and the method further comprising, prior to step (b), the additional step of contacting the at least one synthetic zeolite with a water-soluble potassium salt. This step may be carried out prior to step (a), or it may be carried out between steps (a) and (b).
In another preferred embodiment of the invention, the calcined zeolite is contacted with a source of lithium ions in step (c). Preferably, in this embodiment the lithium ions are in the form of lithium hydroxide or a precursor thereof. Alternatively, the reaction may be carried out by means of solid-state exchange at a temperature in the range of about 100 to about 550xc2x0 C.
In another preferred embodiment of the invention, the zeolite contains sodium ions and further comprises between steps (b) and (c) the additional step of contacting the calcined zeolite with a water soluble potassium salt.